The invention relates to method and apparatus for controlling moving material and the like, and, more particularly, to improved method and apparatus for achieving a normal operating path of material edges during movement of the material in a desired path of travel.
It is a common practice in manufacturing operations involving indefinite lengths of materials such as fabrics to transport the fabric in a desired path of travel for processing and/or collection. Typically, in textile manufacturing operations, textile fabrics in indefinite length form are longitudinally moved in a path of travel for treatment, inspection, and/or collection in roll form. In such operations, it is generally desirable that the fabric be secured close to its edges for effective processing and minimization of waste.
In the general operation of tenter frames, for example, fabric enters the machine at one end where it is engaged along its edges, or selvages, by a series of upwardly extending pins which penetrate the fabric material. The pins are, in turn, secured to and supported by two endless tenter chains. The fabric is thus held along its opposite edges by the upwardly extending pins for the full period of travel through the tenter frame where it may be subjected to various treatments, for example washing, drying, and dyeing.
Typically, tenter frames transversely maintain the fabric as it moves longitudinally through its path of travel. The selvages are typically reenforced to prevent the fabric edges from tearing where they are secured by the pins.
It is also desirable to secure the fabric at the selvages to reduce waste, since the selvage areas must often be trimmed from the finished fabric. One difficulty with consistently pinning the fabric at the selvages is that the fabric may transversely shift as it enters the machine. Additionally, a fabric edge itself may be uneven. If the tenter frame fails to pin an area of the fabric, a gap is produced, particularly when the fabric is stretched downstream. These gaps must be cut out from the finished material, causing waste, inefficient product handling, and possibly machine down time.
Thus, while on one hand it is desirable to pin the fabric as close to the edge as possible to prevent trimming waste, it is on the other hand desirable to ensure pinning of the fabric to avoid defects inherent from mispinning.
Accordingly, the two tenter frame endless chains are typically at least partially supported on corresponding frame members that are transversely adjustable with respect to the path of travel of the moving fabric. These frame members may be adjusted according to the variation of the fabric edge, attempting to secure the fabric near its edges while avoiding mispins.
To control the transverse adjustment of these frame members, it is known to monitor the transverse position of the edges upstream from the point at which they are secured by the frame members and to adjust the frame members to compensate for position variations. Typical control systems employ photosensor devices configured to detect the presence or absence of a corresponding fabric edge.
One difficulty arising from the use of such systems results from color variation of fabrics often run on tenter frames. Color effects may be such that a control system might react too slowly, or not at all, when fabrics of certain colors are run on the tenter frame. Thus, for example, a tenter frame may tend to pin a lighter color fabric nearer its edge than a darker color.
One method of alleviating such problems associated with fabric color is to adjust the intensity of light emitted by the photosensor light source according to fabric color. Such a practice may be inefficient if such adjustments are frequently required. Additionally, light intensity increases may cause control system malfunctions due to reflections from background objects.
Another difficulty encountered with such control systems is that dust and fabric particles frequently collect on photosensor surfaces. This may inhibit photosensor operation and decrease the control system's effectiveness.
Furthermore, typical fabric edges are relatively uneven and may have strings and other abnormalities. Typical tenter frame control devices attempt to adjust the frame members according to the position of the edge. The edge variations may cause unnecessary and overly frequent frame member adjustment.
Furthermore, control systems generally adjust the speed at which corrections are made to frame member position according to the distance the edge deviates from a desired position. Such systems typically vary the correction speed proportionally to the position of the edge within the detection area of one or more photosensors. For example, frame member correction speed may increase as the fabric edge moves across the detection area. Greater speed variation may be achieved by adding photosensors on either side of the fabric edge desired position.
Such control systems may not react fast enough in response to gross fabric edge deviations. Gross deviations are substantial deviations of the fabric edge, most often in the inward direction. They may be caused by tears, particularly at seams, that require a relatively fast response of the frame to avoid a mispin. If the control system is configured to increase speed quickly as the fabric edge moves away from the desired position, the frame member may also move quickly as the edge returns, potentially causing the frame member to overshoot the desired position.